Move sensitive configuration options into separate files

[davehunt]

At the moment usernames and keys for services like Sauce Labs, BrowserStack and TestingBot can either be set via environment variables or in the shared pytest configuration file. Using the configuration file means both private and public options are shared. It can also be difficult to manage when the configuration file is checked into source control and you don't want to expose your credentials to others.

I think it makes sense to add support for new configuration files such as .saucelabs, .browserstack, and .testingbot to hold these sensitive values. These should be able to be stored in the project directory (and can easily be ignored from version control), any parent directory, or the user's home directory. The files should be discovered in this order and the first match should take priority, allowing projects to override user defaults.